The evolution of bite force in horned lizards: the influence of dietary specialization

Abstract

Dietary specialization is an important driver of the morphology and performance of the feeding system in many organisms, yet the evolution of phenotypic specialization has only rarely been examined within a species complex. Horned lizards are considered primarily myrmecophagous (ant eating), but variation in diet among the 17 species of horned lizards (Phrynosoma) makes them an ideal group to examine the relationship between dietary specialization and the resultant morphological and functional changes of the feeding system. In this study, we perform a detailed analysis of the jaw adductor musculature and use a biomechanical model validated with in vivo bite force data to examine the evolution of bite force in Phrynosoma. Our model simulations demonstrate that bite force varies predictably with respect to the gape angle and bite position along the tooth row, with maximal bite forces being attained at lower gape angles and at the posterior tooth positions. Maximal bite forces vary considerably among horned lizards, with highly myrmecophagous species exhibiting very low bite forces. In contrast, members of the short-horned lizard clade are able to bite considerably harder than even closely related dietary generalists. This group appears to be built for performing crushing bites and may represent a divergent morphology adapted for eating hard prey items. The evolutionary loss of processing morphology (teeth, jaw and muscle reduction) and bite force in ant specialists may be a response to the lack of prey processing rather than a functional adaptation per se.

The shadow of the past: Convergence of young and old South American desert lizards as measured by head shape traits

“…Although toad-like head shapes with enlarged ciliaries in "Phrynosauroid" lizards have been mentioned together as convergent traits for living in desert conditions (Valladares, 2004), they may reflect different aspects of their natural histories. Toad-like head shapes may be related to a diet composed largely of small preys, as documented for North American Phrynosoma lizards (Meyers, Herrel, & Nishikawa, 2006;Meyers, Nishikawa, & Herrel, 2018), in comparison with other desert living and hard-preyed specialist lizards of the genera Gambelia and Crotaphytus (Modlin, 2018). However, dietary data for C. adspersa and putatively convergent Liolaemus are limited.…”

Joint angular excursions during cyclical behaviors differ between tetrapod feeding and locomotor systems

“…The function of oral food processingbiting and chewingis to fracture the food substrate, facilitating digestive efficiency and formation of food boluses with the material properties and small size needed to make them safe to swallow (Iriarte-Díaz et al, 2011;Prinz and Lucas, 1997). The size of the food item is also important at the start of chewing because most tetrapod jaws function as third class levers [as third class levers, the jaw-elevator muscle resultant lies between the jaw joint and the bite point; this trade-off also occurs in the case of second class lever arrangements, which may be present in some mammals (Turnball, 1970)] and the ability of the jaw-closing muscles to produce force depends on their lengthtension relationships, a combination that creates trade-offs between bite force and jaw gape that are well documented in lizards, mammals and fish (Dumont and Herrel, 2003;Eng et al, 2009;Gidmark et al, 2013;Herrel and O'Reilly, 2005;Hylander, 2013;Meyers et al, 2018;Ross and Iriarte-Diaz, 2014;Ross et al, 2018;Santana, 2016;Williams et al, 2009). Hence, chewing is often preceded by ingestive behaviors [here we refer to ingestion sensu stricto, the introduction of food into the oral cavity (Hiiemae and Crompton, 1985;Ross and Iriarte-Diaz, 2014)], such as incisor biting and cropping, which function in part to reduce the food substrate to sizes and shapes that allow toothfood-tooth and/or tooth-tooth contact, usually at low jaw gape angles Hiiemae and Crompton, 1985;Hiiemae and Kay, 1973;Reed and Ross, 2010).…”

Correlation of skull morphology and bite force in a bird-eating bat (Ia io; Vespertilionidae)

“…In sigmodontine rodents, strong biters have a more compact and robust skulls and mandibles, whereas weaker biters are more gracile and elongate skulls [12]. Bite force in horned lizards (Phrynosoma) was also associated with morphological changes in jaw and head [73]. Also, in phyllostomid bats, the different bite forces in various species correspond to specific skull structures, where those species with shorter and taller craniums and mandibles possess a stronger bite force [28].…”